Films used for wrapping articles such as consumer goods, for example books, records, compact discs, video cassettes, games, boxes etc., or food products such as fruits, pizzas, chocolate boxes etc., must be provided with a number of properties in order to meet the package requirements in terms of integrity and attractiveness. These properties mainly are
excellent optical properties, i.e. high gloss, high clarity and low haze
a balanced shrinkage in cross-direction and machine direction at low temperature and
a low shrink tension.
Polyvinylchloride (PVC) films meet these requirements and are used to a great extent for wrapping consumer products. However, the environmental concerns about the extended use of PVC films are increasing and the toxic fumes emitted during the sealing of the PVC films can corrode the sealing bars. Therefore, the skilled artisans have tried to replace PVC films with other films having similar properties but causing less environmental concerns. Much research efforts have been spent on ethylene polymer films. Biaxially oriented tubular polyethylene films were found to usually have the required properties for wrapping articles.
U.S. Pat. Nos. 4,354,997 and 4,463,153 disclose a process for producing biaxially oriented blown polyethylene films by biaxially stretching non-stretched tubular films produced from an ethylene polymer, such as low density polyethylene, linear low density polyethylene or a copolymer of ethylene as a main component and an alpha-olefinically unsaturated monomer. The non-stretched tubular film is expanded and extended under particular conditions at a special temperature profile simultaneously in the cross-direction and in the machine direction by means of an internal air pressure.
European Patent Application 0 240 705 relates to tubular heat shrinkable, biaxially stretched, blown films of a mixture of (A) 90 to 50 percent by weight of a linear ethylene/alpha-olefin copolymer having a density of 0.90 to 0.93 g/cm.sup.3 at 25.degree. C. and (B) 10 to 50 percent by weight of an ethylene polymer having a density of 0.87 to 0.91 g/cm.sup.3 at 25.degree. C. and less than the density of the copolymer (A) by at least 0.014 g/cm.sup.3. The non-stretched tubular film is expanded and stretched and, at the same time, biaxially oriented simultaneously in the cross-direction and machine direction by means of an air pressure in the tube.
British Patent specification 866,820 relates to films made of high or low pressure polyethylene, polypropylene or ethylene/propylene copolymers. The polymeric material which has been oriented by uni-axial or bi-axial stretching during its production is first subjected to high energy ionising irradiation. Then the material is heated and uni- or bi-axially oriented and finally cooled. A tubular film is used as a starting material for the first biaxial orientation. Due to the irradiation, the high temperature tensile strength of the film is increased. The biaxial orientation after irradiation is carried out by inflating the film with air or other gas or with a liquid to form a bubble. According to this bubble technique a blown film is produced which is simultaneously oriented in the cross-direction and in the machine direction.
U.S. Pat. No. 4,680,207 relates to a biaxially oriented linear low density polyethylene film which has been stretched in the cross-direction at a stretching ratio of greater than 1 to less than 3 and in the machine direction at a stretching ratio of less than 6 but greater than the stretching ratio in the cross-direction. The film is produced as follows: a molten linear low density polyethylene which is optionally mixed with a low density non-linear polyethylene resin is extruded, blown to a film and biaxially oriented in special equipment. The film is useful for producing tubular and heavy duty shipping sacks.
For producing shrinkable films having high optical clarity, good shrink properties and good mechanical properties British Patent specification 2,097,324 suggests a film made by stretching a film made of the following homogeneous polymeric composition:
(1) 5 to 100 weight percent of a linear copolymer of ethylene with at least one C.sub.8 -C.sub.18 -alpha-olefin, said polymer having a density of 0.900 to 0.940 g/cm.sup.3 and two distinct crystallite melting regions below 128.degree. C. as determined by differential scanning calorimetry (DSC), the temperature difference between those regions being at least 15.degree. C. and PA0 (2) 0 to 95 weight percent of at least one polymer selected from the group consisting of ethylene homopolymers and copolymers of ethylene with an ethylenically unsaturated comonomer, said polymer having only one crystallite melting point below 128.degree. C. PA0 (A) from 75 to 100 percent of at least one linear ethylene/alpha-olefin copolymer having a density of from 0.890 to 0.930 g/cm.sup.3 and PA0 (B) from 25 to 0 percent of at least one high density linear polyethylene having a density of from 0.935 to 0.960 g/cm.sup.3,
The stretching of the film must be carried out within the temperature range defined by the two crystallite melting points of the copolymer (1). The polymers (2) are conventional ethylene homopolymers or copolymers. According to British Patent Specification 2,097,324 "conventional ethylene homopolymers or copolymers" are high density or low density polyethylene made at high pressure or low pressure. According to British Patent Specification 2,097,324 such conventional ethylene polymers having only one crystallite melting point below 128.degree. C. are not useful for producing the shrink films and linear copolymers of ethylene and 1-octene or another alpha-olefin wherein the alpha-olefin comonomer is present in such small amounts that a second DSC peak is not observed are not suitable for producing the shrink films either. The films are produced on a tubular film production line which combines the extrusion and the orientation of the film. British Patent 2,097,324 teaches by reference to U.S. Pat. No. 3,141,912 that the tubular film is oriented by a combination of expansion and longitudinal stretching. The film is biaxially oriented in a continous manner in two mutually perpendicular directions.
The ethylene polymers and polymer compositions disclosed in U.S. Pat. Nos. 4,354,997, 4,463,153 and 4,680,207, in European Patent Application 0 240 705 and in British Patent specifications 866,820 and 2,097,324 are useful for producing films which are simultaneously bioriented in the machine direction and cross-direction. However, only blown tubular films can be bioriented according to the teaching in the above-mentioned publications. A special processing line is required for the biorientation process. The processing line must be able to maintain a permanent gas pressure inside the tubular film. The teaching of the above-mentioned publications is not useful for biorienting cast films or other flat, i.e. non-tubular films.
Flat films such as cast films are usually bioriented in at least two steps, first in one direction and then in the other direction. The inventors have tried to biorient, on a cast film production line, a flat film produced from the polymers disclosed in British Patent Specification 2,097,324. However, the produced films were found to be quite sensitive to minor changes of the processing conditions, such as the stretching temperature, the stretching ratio or the speed of the film production line.
European Patent Application 0212731 suggests the preparation of a mono-axially oriented cast film on the basis of an ethylene homo- or copolymer having a density below 940 kg/m.sup.3. The ethylene polymer may be blended with high density polyethylene or polypropylene and/or different types of ethylene homo- or copolymers having a low density. The use of low density ethylene homopolymers, optionally mixed with minor amounts of other ethylene polymers, for preparing the mono-axially oriented film is exemplified.
It would be desirable to provide a new bioriented film based on ethylene polymers which can be produced from a flat film according to a stretching process involving at least two orientation steps in which the processing conditions, such as temperature and stretching ratio, can be chosen within a relatively broad range and the process parameters may be changed to some extent during the stretching process without a substantial quality loss of the bioriented film. Furthermore, it would be desirable to provide a new process for producing bioriented films from flat films.